The present invention relates generally to a combustor arrangement for a gas turbine engine and in particular to improvements to gas turbine engine combustors incorporating bled diffusers.
In a typical gas turbine engine compressed air is delivered from a compressor to a combustor where it is mixed with fuel and is burnt within the combustor to produce a high temperature and energy gas stream. This high temperature and energy gas stream then flows into and through a turbine system which extracts energy from the stream to drive the upstream compressors which are drivingly connected to the turbines. The turbines may also extract energy from the gas stream to drive a fan, propeller or other equipment for example an electrical generator.
To achieve stable and efficient combustion of the fuel within the combustor it is important to ensure that there is a suitable air flow within and into the combustor. In particular the velocity of the air exiting the compressor is far too high for combustion to occur. Consequently as the air enters the combustor it must be diffused using a diffuser to reduce its velocity and increase its static pressure. A typical diffuser comprises a diverging duct with an increasing cross section through which the air from the compressor flows. As well as diffusing the air flow from the compressor the diffuser also distributes the air flow across the annular cross section of the combustor.
A problem with such diffusers is that a boundary layer develops adjacent to the walls of the diffuser. The air flow within this boundary layer has a lower velocity than the main flow through the diffuser. The size of the boundary layer increases as the air flows through the diffuser with the result that the airflow from the diffuser has a non uniform cross sectional velocity profile. Such a variation in air flow velocity is undesirable for stable and efficient combustion. A further problem is that the angle of divergence of the diffuser duct, and so rate of diffusion, is limited by the occurrence of separation of the boundary layer at the diffuser wall which induces flow losses. Consequently to achieve a significant amount of diffusion of the air flow and/or to distribute the air flow over a significant combustor cross sectional area a conventional diffuser must be relatively long. The available length for the diffuser however is often limited in modern gas turbine engines. This is a particular problem for modern double annular staged fuel combustor arrangements which have a large cross sectional area and require a uniform cross sectional velocity profile.
To address these problems alternative bled diffuser arrangements have been proposed. In such bled diffusers the boundary layer adjacent to the diffuser duct walls is bled from the diffuser. This reduces the size of the boundary layer so improving the uniformity of the cross sectional velocity profile and allowing greater diffuser duct angles, and so diffusion rates, to be used without boundary layer separation. Such bled diffusers are more efficient and have improved performance as compared to conventional diffusers. Various different types of such bled diffusers exist including vortex diffusers and diffusers with perforated duct walls.
Unfortunately the air bled from the diffuser is at a relatively high pressure having been compressed by the compressor. By bleeding it from the main flow the overall efficiency and performance of the gas turbine engine as a whole is reduced. Consequently the improvement of a bled diffuser efficiency and performance is often offset or even outweighed by the loss in overall efficiency and performance of the engine as a whole.
It is therefore desirable to provide a combustor arrangement for a gas turbine engine in which the performance benefit of a bled diffuser can be utilised without significantly affecting the overall performance of the gas turbine engine as a whole and/or which offers improvements generally.
According to the present invention there is provided a combustor arrangement for a gas turbine engine comprising a combustion chamber, fuel nozzles, and a bled diffuser located upstream of said combustion chamber to, in use, direct an airflow from an upstream compressor into the combustor with the fuel nozzles arranged in use to supply fuel into the combustion chamber where it is mixed and combusted with the airflow from the compressor, the bled diffuser adapted to bleed off a portion of said airflow from a main airflow into the combustion chamber; characterised in that at least one bleed duct is connected to the bled diffuser to, in use, return and direct air bled from the diffuser to a main gas flow through the engine at a location downstream of the fuel nozzles.
Preferably the at least one bleed duct is arranged to supply the air bled from the diffuser to a part of the gas turbine engine downstream of the fuel nozzles so that, in use, the air bled from the diffuser provides cooling of said part of the gas turbine engine.
The combustor may be disposed upstream of a turbine of a gas turbine engine, the at least one bleed duct connected to the turbine to, in use, return and direct the air bled from the diffuser to the main gas flow through the turbine.
Preferably at the downstream end of the combustion chamber there is an array of outlet guide vane, within each vane of the array internal cooling passages are defined which exhaust into the main airflow, the at least one bleed duct interconnects the bled diffuser with the internal cooling passages of said vanes so that in use air bled from the diffuser exhausts into the main gas flow through the internal vane cooling passages. The internal cooling passages may be defined in an aerofoil portion of the vane. The vane may comprise a platform and aerofoil and the internal cooling passages may be defined in the platform of the vane.
Furthermore the internal cooling passages exhaust adjacent to a downstream portion of the vanes.
Preferably the at least one bleed duct is located radially inwardly of the combustion chamber. Alternatively the at least one bleed duct is located radially outwardly of the combustion chamber.
The bled diffuser may comprise a vortex controlled bled diffuser.
Preferably the combustor is of a staged combustor type.
The bled diffuser may be defined by radially inner and outer diffuser duct walls and, in use, the main airflow flows between these inner and outer diffuser duct walls, at least one opening is defined in each of the diffuser duct walls through which, in use, air is bled. The at least one duct extends between the inner and outer diffuser duct walls to, in use, interconnect the air bled through the openings defined in each of the diffuser duct walls. Alternatively the at least one bleed duct comprises at least two bleed ducts, the first bleed duct interconnected with the opening in the inner diffuser duct wall and the second bleed duct interconnected with the opening in the outer diffuser duct wall.
The combustor arrangement may further comprise a combustor casing which at least in part is of a double walled construction defining the at least one bleed duct.